The present invention relates to an apparatus for detecting the amount of intake air in an internal combustion engine equipped with a variable intake air control mechanism capable of varying the operating condition of an intake air control valve that controls the flow of intake air into a combustion chamber.
In an internal combustion engine, the amount of intake air to the engine must be detected in order to determine the amount of fuel to be injected. Known methods of detection include the mass flow method and the speed density method. The mass flow method directly measures the amount of air flowing through an intake passage by using an air flow meter. However, when the engine is in a transient condition, the amount of air measured by the air flow meter does not accurately represents the amount of intake air flowing into the engine.
That is, in an accelerating condition in which the throttle valve is being opened from its closed position, not only the amount of air filled into the combustion chamber, but the amount of air filled into the intake passage from the throttle valve to the combustion chamber is also measured; on the other hand, in a decelerating condition in which the throttle valve is being closed from an open position, the fuel filled into the intake passage from the throttle valve to the combustion chamber, and to be introduced into the combustion chamber, is not measured. To correct the measured value of the air flow meter in a transient condition to represent the actual amount of intake air flowing into the engine, processing called xe2x80x9csmoothing xe2x80x9dis performed to smooth the measure value of the air flow meter (refer, for example, to Japanese Unexamined Pat. No. 63-36038). In this processing, a smoothing factor predetermined according to the type of engine is usually used.
In recent years, a variety of variable valve mechanisms have been developed and commercially implemented to achieve optimum valve characteristics according to varying driving conditions. The valve characteristics include valve timing, valve open period (cam operating period), valve lift amount (cam lift amount), etc. Various types of variable valve systems have been devised and include: one that varies the center time of the valve open period while maintaining the valve open period constant in length, by varying the phase of camshaft rotation (the camshaft phase) relative to the crankshaft either continuously or in two steps (ON/OFF control); one that varies the valve open period and valve lift amount in steps while keeping the center time of the valve open period fixed, by using a plurality of cams having different profiles and by switching between them in operation; and one that employs a three-dimensional cam whose profile changes in the axial direction of the camshaft, and that varies the valve open period and valve lift amount continuously by moving the camshaft along the axial direction, while keeping the center time of the valve open period fixed.
In an engine equipped with such a variable valve mechanism, however, the volumetric efficiency (the pumping performance of the engine, expressed as the ratio of the volume of fresh intake air, converted to a value represented by inlet temperature and pressure, to the cylinder capacity) varies during the operation of the mechanism; therefore, with the smoothing process using a fixed smoothing factor, the amount of engine intake air cannot be calculated accurately. This poses a barrier to improving the accuracy of air-fuel ratio control.
Other types of commercially implemented internal combustion engines include one that enhances the intake inertia effect by varying the area or length of the intake passage by means of a control valve installed in the intake passage, and one that enhances combustion efficiency by causing a swirl of a mixture in the intake port around the intake valve by means of a swirl control valve, and by introducing the swirling mixture directly into the combustion chamber. In internal combustion engines that change the intake air flow (flow rate and flow direction) using such a control valve or a swirl control valve, since the volumetric efficiency varies according to the valve operating condition, such engines also have the same problem as that of the internal combustion engines equipped with variable valve mechanisms.
The intake and exhaust valves associated with the variable valve mechanism, the control valve used to vary the area or length of the intake air passage, the swirl control valve, etc. can all be considered as intake air control valves that control the flow of intake air into the combustion chamber.
In view of the above situation, it is an object of the present invention to further improve the accuracy of air-fuel ratio control and, hence, to contribute to the prevention of air pollution, by providing an intake air amount detection apparatus capable of accurately measuring the amount of engine intake air based on the amount of flowing air measured by an air flow meter in an internal combustion engine equipped with an intake air control valve that controls the flow of intake air into the combustion chamber.
To achieve the above object, according to the present invention, there is provided an apparatus for detecting the amount of intake air in an internal combustion engine having an intake passage, a throttle valve installed in the intake passage for controlling the amount of intake air, and a variable intake air control mechanism capable of varying the operating condition of an intake air control valve that controls the flow of intake air into a combustion chamber, the intake air amount detection apparatus comprising: flowing air amount measuring means for measuring the flow rate of air flowing through the intake passage; intake air control valve operating condition detecting means for detecting the operating condition of the intake air control valve; smoothing factor computing means for computing a smoothing factor in such a manner that the smoothing factor is reduced to increase the degree of smoothing when volumetric efficiency decreases depending on the operating condition of the intake air control valve detected by the intake air control valve operating condition detecting means; and intake air amount computing means for computing the amount of engine intake air by smoothing the flow rate measured by the flowing air amount measuring means based on the smoothing factor computed by the smoothing factor computing means.
Preferably, according to the present invention, the smoothing factor computing means computes the volumetric efficiency based not only on the operating condition of the intake air control valve but also on engine load and engine speed, and computes the smoothing factor based on the thus computed volumetric efficiency.
Preferably, according to the present invention, the variable intake air control mechanism is a variable valve mechanism that utilizes an intake valve or an exhaust valve as the intake air control valve and varies the open condition of the valve.
Preferably, according to the present invention, the variable valve mechanism varies intake valve timing by continuously varying the phase of rotation of an intake camshaft relative to a crankshaft within a predetermined angle range.
Preferably, according to the present invention, the variable valve mechanism is a cam switching type mechanism which switches operation between a plurality of cams having different valve opening characteristics.
In the thus constructed intake air amount detection apparatus for an internal combustion engine according to the present invention, since the amount of engine intake air is computed by smoothing the measured value of the flow rate of air flowing through the intake passage in the engine by using the smoothing factor determined in accordance with the operating condition of the intake air control valve, the amount of intake air can be detected with higher accuracy.